Composites of spunbonded nonwoven webs and carded nonwoven webs are known for a wide variety of end uses such as wipes, diaper and hygienic product coverstock, protective apparel applications, and the like.
Spunbonded polymeric nonwoven webs can be produced by extruding polymer through a die to form a multiplicity of continuous thermoplastic polymer strands as the polymer exits holes in the die in a generally downward direction onto a moving surface where the extruded strands are collected in a randomly distributed fashion. The randomly distributed strands are subsequently bonded together by thermobonding or by needlepunching to provide sufficient integrity in a resulting nonwoven web of continuous fibers. One method of producing spunbonded nonwoven webs is disclosed in U.S. Pat. No. 4,340,563. Spunbonded webs are characterized by a relatively high strength/weight ratio, high porosity and abrasion resistance properties and are typically non-uniform in such properties as basis weight and coverage.
Carded polymeric nonwoven webs can be produced by a process wherein voluminous masses of staple polymeric fibers are separated by opposed moving beds of closely spaced needles into individual fibers, aligned for the most part in the machine direction and formed into a coherent web.
Self-bonded, fibrous, nonwoven web and carded web composites are generally disclosed in U.S. patent application Ser. No. 556,353, filed on Jul. 20, 1990, in the name of Geraldine M. Eaton, et al., and self-bonded, fibrous, nonwoven web and meltblown composites are disclosed in U.S. patent application Ser. No. 556,354, filed on Jul. 20, 1990, in the name of Paul N. Antonacci, et al., both commonly assigned to the present assignee. No specific details of construction or end use are described for the composites of this invention in those applications.
A major limitation of multi-layer composite laminates of spunbonded and carded nonwoven webs is that the spunbonded web used to impart strength to the carded web layer, especially in the cross-machine direction, is nonuniform in coverage and basis weight. In many applications, attempts are made to compensate for the poor fabric aesthetics and limiting physical properties that result from this nonuniformity of coverage and basis weight by using webs having a greater number of filaments and a heavier basis weight than would normally be required for the particular application if the web had a more uniform coverage and basis weight. This, of course, adds to the cost of the composite product and contributes to greater stiffness and other undesirable composite features.
In view of the limitations of spunbonded nonwoven webs in multi-layer nonwoven spunbonded and carded web composites, there is a need for improved nonwoven web composites and, particularly, those wherein at least one layer of a self-bonded, fibrous nonwoven web having very uniform basis weight and balanced physical properties is adhered to at least one layer of a carded web of staple fibers.
U.S. Pat. No. 4,275,105 discloses absorbent, nonwoven, stabilized webs wherein a carded web of staple length fibers of rayon having a basis weight in the range of about 20 to about 75 g/m.sup.2 is bonded to spunbonded webs having basis weights of about 5 to about 25 g/m.sup.2 made from polyethylene, polypropylene, ethyl vinyl acetate, ethyl methyl acrylate, polyester, nylon or polyurethane.
U.S. Pat. No. 4,832,852 discloses composites formed by needle punching top and bottom layers of carded webs of cotton to an intermediate layer of a spunbonded synthetic material used in a method of removing oil from a surface contaminated with oil comprising spreading an elongated mat of the composite material on the surface to absorb the oil.
U.S. Pat. No. 4,824,498 discloses laminates used as carpet underlay. In one embodiment, a layer of a spunbonded nylon scrim is adhesively attached to a layer of a carded web of rayon.
U.S. Pat. No. 4,850,990 discloses a disposable diaper with a liquid permeable bodyside liner having a top layer of a pattern bonded, spunbonded web of synthetic fiber such as polypropylene, polyester and the like and a bottom layer of a carded web of polyester/polypropylene fiber bonded together by a thermal or ultrasonic bonding process.
Disposable diaper and hygienic products generally comprise a fibrous nonwoven interlayer, also referred to herein as coverstock, which contacts the wearer's skin, an intermediate layer of absorbent material and an outer impervious backing sheet. Important characteristics of the coverstock are strikethrough, runoff and rewet. Strikethrough is a measure of the ability of the coverstock to pass moisture into the intermediate layer upon initial contact of the moisture. Runoff is a measure of the ability of the coverstock to allow a fluid to pass through it and be absorbed by the intermediate layer. Rewet is a measure of the tendency of moisture to move back from the intermediate layer through the coverstock after initial wetting. Low rewet is required to keep moisture away from the wearer's skin.
A disadvantage of the multi-layer nonwoven web composite disclosed in the patents above is their rewet properties and cross-machine direction tensile strength per unit of basis weight. Thus, there remains a need for multi-layer nonwoven web composites having improved rewet properties and improved cross-machine direction tensile strength per unit of composite basis weight and for processes to make these composites. It is an object of this invention to provide such composites. It is a further object of this invention to provide multi-layer nonwoven web composites comprising at least one layer of a uniform basis weight self-bonded, fibrous web of thermoplastic filaments comprising polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, polyamides, polyesters, blends of polypropylene and linear low density polyethylene and at least one layer of a carded web of staple fibers wherein the staple fibers comprise cotton, polypropylene, blends of polypropylene and polybutenes and blends of polypropylene and linear low density polyethylene. A still further object of this invention is to provide processes for preparing these composites. Other objects of this invention will be apparent to persons skilled in the art from the following description and claims.
We have found that the objects of this invention can be attained by providing nonwoven web composites comprising at least one layer of a uniform basis weight, self-bonded, fibrous, nonwoven web comprising a plurality of substantially randomly disposed, substantially continuous thermoplastic filaments wherein the web has a Basis Weight Uniformity Index (BWUI) of 1.0.+-.0.05 determined from average basis weights having standard deviations of less than 10%, adhered to at least one layer of a carded web of staple fibers. With the greater uniformity of basis weight, coverage and tensile strength made available from the self-bonded nonwoven web having a BWUI of 1.0.+-.0.05 determined from average basis weights having standard deviations of less than 10%, lighter basis weight multi-layer nonwoven composites are attainable.
Among the advantages produced by the multi-layered nonwoven web composites and process for producing same of the present invention are improved rewet properties and improved cross-machine direction tensile strength per unit of composite basis weight as compared to carded web. This improvement is achieved due to the very uniform basis weight property of the self-bonded, fibrous nonwoven webs comprising substantially continuous polymeric filaments which enables lower basis weight self-bonded webs to be used to provide strength to the composites. Improved cross-machine direction tensile strength per unit of composite basis weight is achieved due to the excellent cross-machine direction tensile strength and uniform basis weights exhibited by the self-bonded, fibrous, nonwoven webs used in the present invention. Wetness properties of multi-layer nonwoven web composites can be engineered for specific end uses by varying the hydrophobic or hydrophilic nature of each layer of the composite independently by choice of thermoplastic used or by choice of the treating agent used for each layer such as wetting agents or surfactants. Additionally, the use of blends of polypropylene and polybutene and/or linear low density polyethylene provides the multi-layer nonwoven web composites with better hand and improved softness.